There is proposed an aerobic decomposition treatment in which food waste and domestic wastewater, especially wastewater containing solid matter such as food waste and the like that are crushed by a disposer for disposal, discharged from a collective housing such as a mansion, a hotel, a restaurant, a hospital, a composite building, a food factory and the like, are directly sent to a wastewater tank 32 of a solid-liquid separation tank 14 by a pump 6; the solid matter in the wastewater is collected by a solid-liquid separation device 33 installed therein; aerobic microorganisms, a neutralizing agent, a deodorant agent, and the like are added to the collected solid matter 38; and the collected solid matter 38 is changed to inorganic materials, such as carbon dioxide, water, nitrate ions, sulfate ions, and the like, by the aerobic microorganisms, while being stirred and mixed by a stirring device 17 in a decomposition tank 11. Further, in a wastewater treatment device such as this, by sending air bubbles for aeration to a treatment tank (an aeration tank) by using a blower, decomposition treatment by the aerobic microorganisms in the treatment tank is activated (e.g., see Patent Literature 1).